1. Field of the Invention
The present invention relates to a module incorporating a laser diode, a laser apparatus having the module mounted therein, and a laser processing apparatus.
2. Description of the Related Art
A high-power laser diode is used as a light source for exciting a solid-state laser and a light source for laser-beam processing. The efficiency of conversion from electrical energy to optical energy to stimulate a laser diode is as high as about 50%. When a laser beam is output from the laser diode, therefore, heat as high as or higher than the optical energy of the laser beam is generated in the laser diode. For example, the laser diode that generates light of 50 W generates heat of 50 W or higher. As the temperature of the laser diode rises, the electricity-to-light conversion efficiency falls, and the emission life is shortening. Further, as the temperature shifts the oscillation wavelength, the temperature rise of the laser diode stands in the way of causing light excitation. This requires that some means of suppressing a temperature rise originated from generated heat should be taken in the laser diode.
A module incorporating the laser diode is mainly comprised of three components; a laser diode, a heat sink which cools down the laser diode and electrodes which energize the laser diode. The heat sink often serves as an electrode of the laser diode.
Recently, a laser diode module which can generate a high-power laser beam is demanded as a light source for efficiently welding, cutting by melting, boring and annealing of materials of metal or so. To meet the needs, achievement of high-power laser diodes has been studied. A laser diode bar which is formed by arranging active regions for generating laser beams sideways in a single chip has been developed as a means of achieving high power.
FIG. 1 is a perspective view showing this laser diode bar 101. The laser diode bar 101 generally has a size of {10 mm (width)}×{1.0 to 1.5 mm (cavity length)}×{100 to 150 μm (thickness)}. The top and bottom sides of the laser diode bar 101 are electrode surfaces 104, and one of the sides of the laser diode bar 101 is an emission side 102 which outputs a laser beam. On the emission side 102, emission regions 103 are arranged in a line in the widthwise direction, and the number and width of the emission regions 103 are designed to be optimized by the necessary output. The current is supplied via the top and bottom electrode surfaces 104, causing the emission regions 103 to emit light. Laser diode bars that have output power of 10 W to 100 W are commercially available. Gallium arsenide (GaAs) is mainly used for the substrate for forming a laser diode bar.
The following describes a conventional laser diode module in which a laser diode that generates high-power light is mounted. FIG. 2 shows a module in which a laser diode bar disclosed in Japanese Patent Laid-Open Publication No. H10-209531 is mounted. The module is a laminated body 201 in which laser diodes each mounted on a heat sink are laminated longitudinally. The module has a basic structure wherein a laser diode bar 203 is mounted via a solder layer to a water-cooled heat sink 202 which also serves as a lower electrode. The upper electrode of the laser diode bar 203 and a metal sheet 205 placed on a rubber sheet 204 provided for insulation to the heat sink 202 are interconnected by bonding wires 206. A coolant is supplied via a coolant passage 207 to the heat sink 202 of each layer. Instead of the bonding wire, ribbon-bonding, a metal plate or a metal film may be used as a wire. There has been proposed a module in which a substrate having about the same thermal expansion coefficient as that of a laser diode is provided between the laser diode and a heat sink.
FIG. 3 shows a laser diode module disclosed in Japanese Patent Laid-Open Publication No. H9-129986. A laser diode 301 is sandwiched by terminal plates 302 and 304 which have about the same thermal expansion coefficients as that of the laser diode 301, and the top and bottom sides of the laser diode 301 are fixed by hard solder layers 303 and 305. The lower terminal plate 302 is fixed to a heat sink 308 by an elastic adhesive or a soft solder layer 306. A lead terminal 307 is connected to the upper and lower terminal plates 302 and 304 for electrical interconnection. As the terminal plate 302 and the laser diode 301 which have approximately the same thermal expansion coefficients, the junction interface is not deteriorated even if the temperature of the laser diode fluctuates up and down. The Japanese Patent Laid-Open Publication No. H9-129986 discloses that because the difference between the thermal expansion coefficients of the bottom side of the terminal plate 302 and the heat sink 308 is relaxed by the elastic adhesive or soft solder layer 306, degrading of the cooling performance of the junction part can be suppressed. A mirror surface layer 310 (reflection layer for laser emission) is provided at a side surface of the laser diode 301. The heat sink 308 is provided with a cooling member 311, so that a coolant 314 is guided to a coolant guide portion 312. The coolant 314 cools down the laser diode.
FIG. 4 shows a laser diode module 401 disclosed in Japanese Patent Laid-Open Publication No. H10-41580. A laser-diode first side 415 is fixed to an inner side 403 of a heat absorber 407 by a first solder 402. A laser-diode second side 416 is fixed to an inner side 405 of a lid 408 by a second solder 404. A laser diode is sandwiched between the heat absorber 407 and the lid 408. A bottom side 406 of the heat absorber 407 is connected to a heat accumulator, and the laser diode is cooled down by thermal conduction of the heat absorber 407. The heat absorber 407 and the lid 408 are formed of plastically deformable metals. An outer side 409 of the heat absorber 407, an outer side 410 of the lid 408 and a bottom side 417 of the lid 408 are not mounted on a solid package body. Even when the laser diode module 401 warps due to thermal expansion, the shapes of the heat absorber 407 and the lid 408 follow up the warping. Accordingly, no deformation occurs and no degrading of the cooling performance of the junction part occurs as disclosed in the publication. A discharge side 411 of the laser diode is level with a top side 413 of the heat absorber 407 and an upper end 414 of the lid 408. A groove 418 for supplying solder at the time of connecting several modules laid side by side is formed in the upper outer surface of the heat absorber 407.
FIG. 5 shows the structure of a laser diode module disclosed in Japanese National Publication of the translated version No. H10-507318. In the module, a laser diode stack 503 that has plural laser diodes and heat canceling sheets sandwiched alternately is mounted between a fixed part 501 which also serves as an upper electrode and a base 502 which also serves as a heat sink and a lower electrode. A spring 504 is intervened between the laser diode stack 503 and the fixed part 501. The laser diode stack 503 is sandwiched between the fixed part 501 and the base 502 via the spring 504. Japanese National Publication of the translated version No. H10-507318 also shows the use of a screw instead of the spring 504. The upper and lower electrodes (the fixed part 501 and base 502) are insulated by an insulating sheet 505. The module is characterized in that no solder is used between the laser diode and the heat canceling sheet and between the laser diode stack 503 and the upper and lower electrodes, and the electric contact between the laser diode stack 503 and the base 502 or the fixed part 501 is made only by pressing of the spring 504 in the upper and lower directions. Si, SiC or copper tungsten is used for the heat canceling sheet, and the material selected has a higher thermal expansion coefficient than a GaAs substrate which constitute the laser diode. Because the prior art structure does not use a solder, it is easy to assemble. The illustrated example achieves laser oscillation with a pulse of 100 ms.
The prior arts disclosed in the Japanese Patent Laid-Open Publication No. H10-20953, the Japanese Patent Laid-Open Publication No. H9-129986, the Japanese Patent Laid-Open Publication No. H10-41580 and the Japanese National Publication of the translated version No. H10-507318 do not raise problems if the average output power of the laser diode module is less than 10 W. In case of a high-power laser diode bar whose average output power is 20 W or greater, however, there is a high probability that the output power gradually decreases and the electrodes are eventually disconnected. Especially when the ON and OFF states of the oscillation light of the laser diode with an interval of several seconds or so are repeated, the probability of causing power reduction, disconnection and shifting of the oscillation wavelength increases, thereby shortening the service life. To overcome the problems, the laser diode in the module should be cooled down stably over a long period of time. This requires improvements on the following three issues associated with cooling of the laser diode.
(1) Decomposition of the solder layer that connects the heat sink and the laser diode
(2) Decomposition of the contact between the laser diode and the upper electrode
(3) Warping and deformation of the laser diode.
The problem 1 will be explained in detail. Conventionally, a laser diode or a submount substrate on which a laser diode is mounted and which has about the same thermal expansion coefficient as that of the laser diode is mounted on a heat sink using a soft solder. The soft solder is connected to the heat sink and the laser diode or the submount substrate as it is alloyed with the metal of the mount interface, e.g., gold. The alloyed phase is grained and is spotted in an unalloyed soft solder layer. The alloyed phase differs from the unalloyed soft solder layer in thermal expansion coefficient. Given that the temperature of the laser diode when the laser diode is set on with predetermined power is T1 and the temperature of the laser diode when the laser diode is set off is T2, the temperature of the laser diode fluctuates between T1 and T2 at maximum. When the temperature rise and fall are repeated, deformation of the interface between the alloyed phase and the soft solder phase becomes greater, micro cracks would occur at the interface. As the thermal resistance of the portion where the crack occurs becomes higher, the temperature locally rises there. As the temperature rises, the diffusion of metal atoms at the mount interface in the soft solder is accelerated, thus increasing the probability of causing voids due to the Kirkendall effect that is brought about by the growth of the alloyed grain or the metal diffusion. The growth of those cracks or voids further increases the thermal resistance of the whole solder layer, thus increasing the temperature of the laser diode to lower the output power and shifting the oscillation wavelength. Finally, large cracks occur at the junction portion. As the cracks or voids are grown, the laser diode or the submount substrate having the laser diode mounted thereon cannot be connected to the heat sink by a soft solder layer and is partly peeled off. The partial peeling off increases the temperature at the peeled off portion, thereby further accelerating alloying of the vicinity portion and metal diffusion. This makes the portion to be peeled off larger and raises the temperature of the chip, so that the soft solder may be melted and dropped off or oxidized to be insulated from the heat sink. The conventional module shown in FIG. 3 has no means for suppressing the decomposition of the soft solder layer. In the case of a module which does not use a solder as in the prior art in FIG. 5, there is no degrading of the soldered portion so that the problem 1 does not occur, but mere pressing without soldering makes it very difficult to obtain a low thermal resistance between the heat sink and the laser diode which is needed to continuously output light with average power of 10 W or higher.
The problem 2 will be explained in detail below. When the ON-OFF states of the laser diode are repeated frequently, the temperature of the junction portion to the upper electrode of the laser diode fluctuates up and down for the same reason given in the description of the problem 1. The wiring of the upper electrode of the laser diode is conventionally made mainly by bonding wires or a bonding ribbon as shown in FIG. 2, or is achieved as the plate electrode is fused with a solder or thermally compressed. When bonding wires or a bonding ribbon of the same material as the upper electrode of the laser diode is used, a matrix of grains with different crystal orientations is present at the solidified portion due to the influence of the spontaneous fusion solidification at the time of connection, though no alloy is present. When a solder is used, alloying with the surface metal yields an alloyed grain. As the connected portion suffers a large deformation and its temperature fluctuates up and down, cracks are produced in the grain interface due to the anisotropy of thermal expansion or difference in thermal expansion, eventually causing disconnection. An increase in electric resistance originated from the deformation of the connected portion or the increased cracks may generate heat at the connected portion. When the electrode is partly separated, the current is concentrated on the remaining connected portion. This further increases the load on the connected portion, generating heat, so that most of the entire connected portion of the electrode may be separated eventually.
The problem 3 will be explained in detail next. A laser diode is formed by film deposition on a GaAs substrate. Because the film deposition is done on only one side of the substrate, the laser diode does not have a composition symmetry in the thicknesswise direction. This makes a slight difference in thermal expansion coefficient in the thicknesswise direction, so that the laser diode may warp. In the case of the laser diode bar shown in FIG. 1, particularly, as the ratio of the cavity length to the width is large, the laser diode is likely to warp in the widthwise direction along which its length is longer. Even though there is no warping of the laser diode at the time of mounting, the temperature rises during laser output, making the force to cause warping greater. As a result, cracks occur at the junction interface between the laser diode and the heat sink or the submount substrate, producing a thermally-disconnected portion which would heat up the laser diode. This may bring about the problems 1 and 2. In case where the high-power laser diode is sandwiched between the plastically deformable heat absorber and the lid and is cooled from the bottom of the heat absorber as in the prior art in FIG. 4, it is extremely difficult to cool down the heat of 20 W or greater with cooling of the thermoconductive system by the heat absorber while warping of the laser diode is allowed. If the laser diode warps, the direction of light to be output changes, raising an application problem.
In short, when the average output power of the laser diode module becomes high, the structure and mode of the conventional laser diode module suffer a difficulty in obtaining a stable output over a long period of time.